Self-propelled lawn mower

ABSTRACT

The invention concerns a self-propelled lawn mower and a method adapted for using said machine. The machine comprises: a mobile frame ( 1 ) including at least one drive wheel ( 6 ) and at its lower surface at least two juxtaposed cutters ( 3 ), associated each with an electric motor; an electronic system for controlling the forward movement of the machine; an on-board computer; an electric power supply; means for measuring the force supplied by each cutter, said measurement being transmitted to the on-board computer; a memorized algorithm for seeking the grass surfaces to be mowed by the interaction of the machine forward movement system, said force measurements and the on-board computer; means for detecting the limits of the surface to be mowed by a magnetic field measurement or the variation of a magnetic field emitted or caused by a set or buried wire, located at said limits.

This application is the national phase of international applicationPCT/BE98/00038 filed Mar. 18, 1998 which designated the U.S.

This invention relates to improvements to an autonomous lawn mower, moreespecially a multiple-cutting-head autonomous lawn mower.

The European patent EP-A-0 550 473 puts forward a low-power roboticmower traveling almost continuously over the surface to be mowed inrandom manner.

From the same above mentioned patent, a first improvement is known withrespect to a purely random system by utilizing the measurement of theenergy absorbed by the cutting head in order to determine whether or notthe machine is in an already mown area. This enables the mower at oneand the same time to adjust its progression speed in order to remainonly a short time in an already mown area and on the other hand tomodify its behavior when passing over a non-mown area (e.g. then tofollow a spiral mowing path).

This invention puts forward additional improvements for the mowingefficiency of a robotic mower, in particular for mowing large surfaceareas, for example for golf courses or park surface areas.

One of the disadvantages associated with mowing devices designed forlarge surface areas such as utilized at present, consists in theutilization of a relatively powerful cutting system, the surface of thecutting head or heads having to be greater than for classic mowers fordomestic use. The inertia of the cutting disk(s) or blade(s) is alsogreater. The safety in use of these devices therefore presents aproblem, especially if a foreign body accidentally comes into contactwith the cutting head(s). This safety problem also arises, and even moreso, when robotic mowing devices are to be utilized for large surfaceareas.

According to a first aspect of the invention, a robotic mower is putforward which does not have or greatly reduces this inconvenience.

According to a second aspect of the invention, a guiding method for arobotic mower is put forward, more especially adapted to the roboticmower according to the invention.

Fundamentally two basic techniques exist for defining the route of amobile autonomous robot, e.g. a robot for lawn mowing:

a systematic mowing technique by which the mower systematically mows thelawn, as a human being does with a classic mower, either by following apredetermined route, for example by concealing a guiding wire in theground, or by utilizing a mapping and/or position finding system bymeans of markers;

a random route technique by which the robotic mower moves constantly inrandom manner on the surface area to be mowed without searching for adetermined route.

The advantage of the latter technique is that it can be made naturallyself-adapting which gives the system a very great strength. Itsnecessitates on the other hand a great number of passes in order toobtain a complete coverage of the surface area (3 passes guaranteeing97% coverage). This hardly poses any problem in the case of a continuoustechnique (see European patent 0 550 473 relating to a possiblysolar-powered robotic mower), but constitutes a handicap if a lawn is tobe mowed occasionally and quickly with a robotic mower withoutpredetermined guiding system.

All the elements mentioned but not described in detail in thisspecification are analogous or identical to those described in the abovementioned European patent 0 550 473, incorporated by reference into thisspecification.

According to a first aspect of the invention a robotic mowing devicewith several cutting heads is put forward.

More especially the mowing device has the following elements, takenseparately or in any combination:

a mobile chassis comprising at least one driving wheel and on its lowerside at least two juxtaposed cutting heads, preferably each connected toa preferably electric, independent motor

an electronic control system

an on-board computer

an electric power supply unit

a means for measuring the force provided by each cutting head, theaforesaid measurement being transmitted to the on-board computer

an algorithm stored in memory enabling the interaction of theadvancement system of the device, the above mentioned force measurementsand the on-board computer, means capable of enabling for example thesearching for surface areas of grass to be cut;

a means for detecting the boundaries of the mowing surface area bymeasurement of a magnetic field or of the variation of a magnetic fieldemitted or caused by a laid-down or buried wire located at the aforesaidboundaries, and possibly

a means for detecting obstacles by analysis of relative movements of anupper plate of the device elastically integral to its chassis such asdescribed in the European patent EP 0 550 473

an anti-theft device based on the measurement of the movement away fromthe ground and/or on a code peculiar to the user.

According to a first aspect of the invention the adoption of a machinewith several cutting heads, preferably identical, is therefore provided.Thus two, three, four, five or more than five cutting heads can beprovided, e.g. eight cutting heads, installed on one and the same mowerchassis. According to an embodiment concretized at present, five cuttingheads are provided. The cutting heads are preferably situated at thefront of the chassis, which can measure e.g. from 60 cm to 2 m in width,their locations being at least offset one in relation to the other overthe width of the chassis (direction perpendicular to movement). Thepower associated with each cutting head is generally situated between 10and 90 watts, preferably between 15 and 60 watts, more preferentiallybetween 20 and 50 watts. By way of example, the diameter of a cuttinghead can vary from 10 to 50 cm, a diameter of approximately 20 cm havingproved advantageous.

The robotic mower with several cutting heads consists of a chassisdriven by two driving wheels, similar to that described in the abovementioned European patent 0 550 473. The driving wheels willnevertheless advantageously have a considerably larger diameter, e.g.from 2 to 4 times greater, than the other wheels. At the front of themachine several independent cutting heads are mounted. According to anembodiment preferred at present each of the cutting heads is operated byits own electric motor.

These cutting heads are preferably flexibly mounted so that the groundirregularities can lift each cutting head independently. When suchlifting occurs, the cutting power of the corresponding head couldmomentarily no longer be taken into account by the computer (parameterdecoupling).

The cutting heads can be of the type described in the patent applicationPCT WO 96/242432.

The motors of each head can be independently operated and themeasurement of the torque of each motor is transmitted to the on-boardcomputer.

According to an embodiment of the invention, the cutting force of eachhead is constantly analyzed (e.g. several hundred times per second), inknown manner, by the computer of the mower. When the machine partiallyoverlaps a mown area and an unmown area, the result of the analysis ofthe forces on each cutting head enables the computer to situate themachine in relation to the boundary of the two areas and to positionitself in consequence.

The on-board computer can thus control the route of the mower in orderto make it approximately follow the demarcation between the two areas,resulting for example from the trace left by a previous pass of therobotic mower.

In order to mow a determined surface area, the device can be placed andactuated in any location of this surface area. The route can first beeffected randomly until the computer detects a mown area/unmown areademarcation and actuates the above mentioned algorithm. The normalprogression speed of the device can vary but is advantageously ofapproximately 1 m/sec (e.g. 2 to 5 km/h).

Alternatively, when the perimeter of the area to be mowed is defined bya wire, in which for example a low-frequency current flows such asdescribed in the above mentioned patent EP-A-0 550 473, the abovementioned guiding technique based on the aforementioned demarcation canbe combined with a technique linked to a systematic starting of themowing process along the peripheral wire as described hereafter.According to another aspect of the invention, each technique canmoreover also be utilized independently.

According to this second technique, at the start of mowing the machine(with one or several cutting heads) is positioned along the peripheralwire. After starting, the on-board computer periodically, in knownmanner, measures the amplitude of the signal emitted by the peripheralwire. This measurement enables the on-board computer to know thedistance separating it from the wire and therefore to control thedirection of the machine in order to keep it at a fixed distance fromthe wire.

If the length of the wire has previously been stored in the memory ofthe on-board computer, this can determine with a reasonable precisionthe time when a complete round has been effected by the mower along thiswire. The mower can then move away from the wire by a distance equal tothe cutting width in order to be able to effect a new circuit at adistance from the wire increased by the cutting width. The operation canthus be repeated by each time increasing the distance between the mowerand the peripheral wire, ideally until arriving at the center of thearea to be mowed.

According to a variant of the embodiment, it is not necessary to storethe length of the above mentioned wire in the computer. The length canindeed be determined by the on-board computer by integrating thedifferences in speed between the driving wheels of the machine (changesof direction), until the cumulated change reaches or exceeds 360°. Tothis end, the system can also advantageously integrate a magnetic orinertial compass.

It should be understood that this method according to the invention,based on a wire located at the boundaries of the surface area to bemowed can be applied to all autonomous robotic mowers, including thosewhich only have one single cutting head.

Nevertheless, in certain circumstances, this method based on a wirelocated at the boundaries of the surface area to be mowed has twolimitations:

the farther the machine moves away from the wire, the more the signaldecreases and the more difficult it is to obtain a precise positioningin relation to this wire,

moreover, the magnetic field variation is not uniform at all points ofthe route, it is faster for example in the angles.

It is possible therefore advantageously to resort to a combination ofthe two above mentioned techniques: when the positioning precisionbecomes insufficient, the multiple-head robotic mower according to theinvention can adjust its path, due to the analysis of the differentcutting forces of the cutting heads.

Finally, having arrived at a central portion of the ground, where thesignal from the wire is very weak and the gradient practically zero, therobotic mower can therefore only operate, if it has several heads, on amode based on the analysis of the cutting forces of each head, and/orbased on a random path if no precise “mown area/unmown area” boundary isdetected. This latter possibility can be applied for a mower with onesingle cutting head.

Alternatively, in this central portion, the robotic mower, with one orseveral heads, can adopt a systematic mowing system, e.g. either inspiral (see patent EP 550473) or in back and forth mowing.

This latter variant consists in a back and forth movement of the mower,the mower moving in a straight line, possibly guided by means of datafrom a compass, until the signal from the peripheral wire is greaterthan a reference value, in which case the mower turns back with adisplacement of its track of one cutting width in order thus to approachthe other extremity of the weak signal area and again to turn back withdisplacement, e.g. with a displacement corresponding to or less than acutting width.

The mower according to the invention can be powered by one or morerechargeable batteries, the recharging being effected according to knowntechniques for example by providing in the movement algorithm, aperiodic return of the device to a, preferably induction, rechargingstation, such as described in the European patent 0 744 093.

According to another aspect of the invention, in the robotic mower, asource of energy can be provided consisting of a heat engine generator,operating with a classic fuel, of low power driving the robotic unitthrough a regulator. Preferably the generator is the low-voltagethree-phase type, running at a stable speed but higher than that oftraditional generating sets and driving the system via a bridgerectifier. This type of generator is commercially available for examplefor camping or installation in sailboats.

By way of example, the engine can be a 2-stroke one of 35 cm³ driving athree-phase generator. The engine can run at 3000 or 4500 revolutionsper minute thus generating up to 200 or 450 watts respectively. Thethermal generator can thus be derived from a Honda EX 500 generatorprovided with a GE 35 engine, the unit being modified by connecting a 24V 10 A regulator.

This technique enables the implementation of a very compact and lightunit, which produces electricity flexibly, with an autonomy greater thanthat possible by utilizing a rechargeable battery. It should beunderstood that according to an aspect of the invention, a similar typeof power supply can be adapted to all mobile autonomous robots.

According to yet another aspect of the invention, the mower can beprovided with a rechargeable battery connected to the above mentionedthermal generator whether or not suited for supplying electric powerdirect or through the rechargeable battery which is connected thereto.

According to still another aspect of the invention, the mower can beprovided with an ultrasonic detector, of the sonar type, from which thedata are transmitted to the on-board computer in order to enable thedevice to slow down when approaching an obstacle. The obstacle itselfbeing detected by the relative movement of a plate and of the chassis asalready described.

FIG. 1 is a schematic top view of an embodiment of a lawn moweraccording to the invention.

FIG. 2 is a schematic elevation view of the mower from FIG. 1.

FIG. 3 is a view in perspective of the mechanical part of anotherrobotic mower according to the invention.

In FIG. 1 a beam chassis 1 can be seen, covered by a hood 2, e.g. inpolycarbonate or polyester. The beam chassis is mechanically connectedat the front to five identical cutting heads 3 situated on two lines andoffset in relation to the perpendicular to the direction of movement, soas to be able to cover a substantial part of the width of the mower.Three free wheels 5 can be discerned situated in triangle at the front,the foremost free wheel 5′ being a loose wheel. Flexible attachmentmeans 4 for the heads, and two driving wheels 6 driven by electricmotors 7 can also be discerned. These driving wheels will generallyadvantageously be of large diameter in order to avoid all jarring of thechassis in movement. The generator is located in 8, the computer in 9and the optional ultrasonic detector in 10. The device has a width ofthe order e.g. of 1 meter.

In FIG. 3 most of the above mentioned elements can be discerned. Theplate 2 is not represented. The loose wheel from FIG. 1 has beenreplaced by an element in arc of circle capable of pivoting like a loosewheel, arc of radius greater and bearing a set of wheels 11. Thisconfiguration has proved to be advantageous for avoiding too great ajolting of the unit. The two other free wheels 5 are in the form ofshort rollers.

What is claimed is:
 1. An operating method for a robotic lawn mowerwhich comprises: positioning the mower along a wire delimiting theperiphery of the surface area to be mowed, periodically receiving withan on-board computer the measurement of the amplitude of a signalemitted by the peripheral wire, sending commands by the computer to acontrol system in order to make the mower advance at a constant distancefrom the aforesaid peripheral wire, evaluating by the computer on thebasis of the measurement of the distance the time when a complete roundhas been effected by the mower along the wire, and controlling throughthe computer a positioning of the mower at a distance from the aforesaidwire greater than the previous one, the difference being equal too orless than the cutting width.
 2. A method according to claim 1 wherein,when the signal emitted by the wire is of an intensity of less than areference value, the computer applies an algorithm based on a randompath.
 3. A method according to claim 1 wherein, when the signal emittedby the wire is of an intensity of less than a reference value, thecomputer applies an algorithm based on a spiral.
 4. A method accordingto claim 1 wherein, when the signal emitted by the wire is of anintensity of less than a reference value, the computer applies analgorithm based on a back and forth systematic mowing, the change ofdirection being controlled by the level of the signal emitted by theaforesaid wire.
 5. A method according to claim 1 wherein the computerevaluates on the basis of the measurement of the distance combined witha measurement of the changes of direction.
 6. Robotic mowing devicecomprising an on-board computer able to act on an electronic advancementcontrol system said computer being programmed for applying the processof claims
 1. 7. Robotic mowing device according to claim 6 characterisedin that it comprises at least two cutting heads.
 8. Robotic mowingdevice according to claim 7 characterised in that the two cutting headsare each driven by an independent motor.
 9. Device according to claim 7comprising furthermore a means for measuring the force provided by eachcutting head, the aforesaid measurement being transmitted independentlyto the on-board computer able to act on the electronic advancementcontrol system.
 10. Device according to claim 6 further characterised inthat it is programmed for applying a mowing process—when the signalemitted by the wire is of an intensity of less than a referencevalue—based on a random path combined with the measurements of the forceregistered in at least two cutting heads.
 11. Device according to claim6 wherein the motors are electrical motors.
 12. Device according toclaim 6 comprising a means for detecting obstacles by relative movementof a plate flexibly mounted on said device.
 13. Device according toclaim 6 comprising a means for measuring a magnetic field or thevariation of a magnetic field emitted or caused by a laid-down or buriedwire located at the aforesaid boundaries.
 14. Device according to claim6 including an electric power supply unit which comprises a heat enginegenerator.
 15. Device according to claim 7 wherein the driving wheelshave a diameter from 2 to 4 times greater than the other wheels. 16.Device according to claim 6 integrating a magnetic or inertial compass.17. Device according to claim 6 wherein the cutting heads are flexiblymounted so that ground irregularities can lift each cutting headindependently.
 18. An operating method for an on-board computer in arobotic lawnmower comprising the steps of: receiving periodically ameasurement of the amplitude of a signal emitted by a peripheral wire,sending commands to a control system within said robotic lawnmower inorder to make the mower advance at a constant distance from theaforesaid peripheral wire, the lawn being then cut on a defined width,evaluating, on the basis of the measurement of the distance, the timewhen a complete round has been effected by the mower along the wire,then controlling a positioning of the mower at a distance from theaforesaid wire greater than the previous one.
 19. An operating methodaccording to claim 18 wherein the difference in the distance from thewire is equal to or less than the said cutting width.
 20. An operatingmethod according to claim 18 wherein the time when a complete round hasbeen effected is also evaluated on the basis of a measurement of thechanges of direction.
 21. A method according to claim 18 wherein, whenthe signal emitted by the wire is of an intensity of less than areference value, the computer applies an algorithm based on a randompath.
 22. A method according to claim 18 wherein, when the signalemitted by the wire is of an intensity of less than a reference value,the computer applies an algorithm based on a spiral.
 23. A methodaccording to claim 18 wherein, when the signal emitted by the wire is ofan intensity of less than a reference value, the computer applies analgorithm based on a back and forth systematic mowing, the change ofdirection being controlled by the level of the signal emitted by theaforesaid wire.
 24. A method according to claim 18 wherein the roboticlawnmower has two cutting heads each driven by an independent motor andcomprises a means for measuring the force provided by each cutting head,the aforesaid measurement being transmitted independently to theon-board computer and being the basis to operate the electronicadvancement control system when the signal emitted by the wire is of anintensity of less than a reference value.
 25. A method according toclaim 18 wherein the lawnmower further comprises a magnetic or inertialcompass.
 26. An operating method for an on-board computer in a roboticlawnmower comprising the steps of: receiving periodically themeasurement of the amplitude of a signal emitted by the peripheral wire,sending commands to a control system in order to make the mower advanceat a constant distance from the aforesaid peripheral wire, the lawnbeing then cut on a defined width, evaluating, on the basis of themeasurement of the change of direction, the time when a complete roundhas been effected by the mower along the wire, then controlling apositioning of the mower at a distance from the aforesaid wire greaterthan the previous one.
 27. A method according to claim 26 wherein, whenthe signal emitted by the wire is of an intensity of less than areference value, the computer applies an algorithm based on a randompath.
 28. A method according to claim 26 wherein, when the signalemitted by the wire is of an intensity of less than a reference value,the computer applies an algorithm based on a spiral.
 29. A methodaccording to claim 26 wherein, when the signal emitted by the wire is ofan intensity of less than a reference value, the computer applies analgorithm based on a back and forth systematic mowing, the change ofdirection being controlled by the level of the signal emitted by theaforesaid wire.
 30. A method according to claim 26 wherein the roboticlawnmower has two cutting heads each driven by an independent motor andcomprises a means for measuring the force provided by each cutting head,the aforesaid measurement being transmitted independently to theon-board computer and being the basis to operate the electronicadvancement control system when the signal emitted by the wire is of anintensity of less than a reference value.
 31. A method according toclaim 26 wherein the measurement of the change of direction is performedby integrating the differences in speed between driving wheels operatingthe lawnmower.
 32. An operating method according to claim 26 wherein thedifference in the distances from the wire is equal to or less than thesaid cutting width.